Wireless communication networks are often organized into a plurality of logical communication channels over which messages can be transmitted simultaneously. In some cases, the communication channels are configured as multiple access channels, so that they function as subnetworks, also referred to as “subnets,” or simply as “nets.”
Providing a plurality of subnets within a multi-access wireless network can be a powerful approach for enabling efficient communications and collaborations, for example by dedicating specific subnets to be used for specific purposes. Nevertheless, it is inevitable that some subnets may be more heavily used than others, thereby leading to inefficient utilization of the available bandwidth as a whole.
Strategies that are employed to enable simultaneous communication in a wireless network include Frequency Division Multiple Access, or “FDMA,” Time Division Multiple Access, (“TDMA”), and Code Division Multiple Access (“CDMA”), among others. FDMA is a strategy whereby different channels or subnets communicate simultaneously on different frequencies.
TDMA enables simultaneous communication over a given frequency by defining a plurality of timeslots during which message segments can be transmitted, where the timeslots are repeated once every epoch. In a more general case, FDMA and TDMA can be combined so as to create a network architecture in which a different frequency is assigned to each of a plurality of subnets during each time slot, so that, from the perspective of a given subnet, each epoch comprises a contiguous series of timeslots that may, or may not, vary in frequency over the course of an epoch. In situations where it is desirable to protect the network against eavesdropping and/or malicious interference, the timeslots assigned to each subnet can be configured to “hop” in frequency during each epoch in a pseudo-random manner.
Within a given subnet, transmission access to the available timeslots can be controlled in either of two ways. If message traffic is relatively light, it may be advantageous to implement a “contention” protocol, whereby nodes transmit messages in arbitrarily selected timeslots whenever a need arises. Typically, any messages that are not received due to message “collisions” are retransmitted.
When message traffic on a given subnet is heavier, it may be more efficient to assign or “dedicate” message transmission timeslots to the member nodes in a subnet, so that collisions are avoided. According to this approach, each member node maintains a transmission queue, which may be prioritized according to different levels of urgency, and transmits messages from the queue only during its assigned timeslots during each epoch.
While these strategies for timeslot allocation can provide efficient usage of the available bandwidth, nevertheless significant message transmission latency can arise due to frequent collisions (in the case of a contention strategy), or due to a wide spacing between transmit timeslots (in the case of a dedicated timeslot strategy). These high latencies can be especially problematic in situations where it is necessary to transmit high priority, time sensitive messages that may be delayed due to a high volume of other, lower priority messages.
An example of a multiuser, multiple access wireless network is the Link 16 networking protocol. Link 16 is a widespread tactical wireless networking protocol that is used by frontline land, air, and naval systems in the United States, NATO, and other allied nations to enable multiple users to share situational awareness data. The protocol is based on an omnidirectional waveform that transmits information in pulses spaced 13 microseconds apart and distributed across 51 frequencies. Link 16 implements a TDMA protocol, whereby information is transmitted in sequential timeslots that repeat every TDMA frame, or “epoch.”
Link 16 also implements FDMA and CDMA, whereby a unique frequency and encoding key is assigned to each subnet for each timeslot in the epoch. Each of the subnets implements either dedicated access, contention access, or “timeslot reallocation” access as the timeslot access mechanism. Timeslot reallocation is essentially a dedicated access strategy that is periodically updated via a collaborative renegotiation by the nodes. For the purposes of this disclosure, timeslot reallocation can be considered to be a form of dedicated timeslot access.
For each implementation of Link 16, the subnets are grouped into functional areas, and allocated to “Network Participation Groups” (NPGs), also sometimes referred to simply as Participation Groups (PGs), which are virtual subnetworks that are designated to carry messages having specific functions and purposes. This strategy of distributing the subnets among functional NPG's allows the Link 16 network designer to make efficient use of the available bandwidth by determining how the bandwidth will be allocated for each functional group in the network. However, it is inevitable that the communication traffic will be much higher on some subnets/NPG's than on others.
A typical link 16 network is shown in FIG. 1. In FIG. 1, the blocks 10 in the single ring 12 represent a series of contiguous timeslots in an epoch that are assigned to a single subnet of the network. The ring 12 is only one logical layer, or subnet, in a logical “stack” 16 of subnets that are included in the network, whereby each timeslot 10 in each epoch is distributed among the plurality of subnets by implementation of FDMA and CDMA. Details of the construction of these Link 16 subnets can be found in MIL-STD-6016, incorporated herein by reference in its entirety for all purposes.
Note that the vertical dimension in the stack of rings 16 does not indicate a simple frequency or code separation between the NPGs or subnets, but is intended only to indicate the logical separation of the subnets. In fact, during each epoch the timeslots assigned to each subnet typically “hop” among various assigned frequencies and coding schemes from one timeslot 10 to the next.
Prior to the start of a mission, the timeslots 10 that have been assigned to each NPG are distributed by a network planner (not shown) among the participant nodes 14 of the NPG as transmit, receive, and relay timeslots. Accordingly, during the course each epoch, each participant 14 in an NPG is given at least one opportunity to transmit a message in an assigned transmit timeslot. If the node has been designated as a relay node, it will also have at least one opportunity during each epoch to relay a message previously received from another node in an assigned relay timeslot. For purposes of this disclosure, a relay timeslot can be considered to be a type of transmit timeslot such that, for a given node 14 in a subnet 12, each timeslot 10 is designated either as a transmit timeslot or a receive timeslot.
Of course, depending upon the implementation, a given participant 14 in an NPG may have more than one transmit timeslot assigned to it during each epoch, and certain users 14 may be given more transmit assignments than other users 14, depending on the expected transmit volume of each of the users 14.
As discussed above, the strategies of contention and retransmission in a contention subnet, and of limiting transmissions to assigned, dedicated timeslots that repeat every epoch in a dedicated timeslot subnet, can lead to significant message transmission latencies, as nodes are forced to wait for retransmissions, or to wait for their assigned transmit timeslots to arrive during each epoch. In Link 16 networks, this latency delay can be several seconds or longer, which can be problematic, if not dangerous, when it is necessary to transmit high priority, time critical messages, for example during a combat mission.
One approach to reducing latency for urgent messages in a subnet is to assign a relatively large number of timeslots during each epoch to nodes of the subnet that perform functions typically requiring low latency. However, in many cases these low latency users do not require high network bandwidth, such that many of these dedicated timeslots go unused and the bandwidth efficiency of the subnet suffers.
What is needed, therefore, is an improved protocol that minimizes latency without sacrificing subnet bandwidth efficiency when transmitting high priority messages over a multiple-access wireless network such as Link 16.